The invention relates to a device and a process in accordance with the preamble of claim 1.
In this respect it must be remembered that it is necessary to set the printed image by optimizing the relative position of the rollers, involved in the inking and printing process, in all areas of rotary printing. Thus, in the case of gravure printing presses the position of the impression roller is set in relation to the printing roller.
In the case of flexographic presses the counter-impression cylinder, the printing roller and the engraved roller are set in relation to each other.
Therefore, there exist flexographic presses that are equipped with a printing roller and an engraved roller that can be moved on at least one bracket of the printing machine frame. These two rollers can be employed by means of their own actuating drive both independently of each other as well as together at the counter-impression cylinder, on which the material web to be printed rests.
Thus, the DE 29 41 521 A1 and DE 37 42 129 A1 show printing machines, in which the bearing blocks of the carriages carrying the printing cylinders are guided in carriage guides of the inking system brackets of the printing machine frame and are provided with their own spindle drives and in which the carriages of the printing cylinders are provided with other carriage guides for the carriages, which carry the bearing blocks of the inking or engraved rollers and which exhibit in turn their own spindle drives.
The DE 40 01 735 A1 discloses a flexographic press, in which the carriages, carrying the printing roller, and the carriages, carrying the inking or engraved rollers, are guided in a common carriage guide of the ink system brackets of the printing machine and are moved jointly and individually by means of spindle drives.
In the case of rotary presses of this known type, the printed image is set in the known way as follows. An electronic controller is provided that can resort to data entered into a storage device. The data relate to the regulating distance between the printing roller and the counter-impression roller in consideration of the geometric dimensions of the machine and the diameter of the rollers. Then this controller adjusts the relative roller position so that it should be guaranteed that all parts of the printed image are transferred.
Of course, the different rollers, printing forms, as well as the material to be printed and all other parts involved exhibit geometric tolerances so that an additional adjustment is often necessary.
This additional adjustment is executed by means of a press guide who adjusts the roller positions while viewing the printed image.
This type of adjustment of the printed image guarantees that with the minimum pressure applied to the rollers involved in the printing process a good printed image is obtained. This type of adjustment of the printed image is, however, complicated, requires a lot of time and rejects and has, furthermore, the drawback that it depends on the subjective judgment of the press guide using visual inspection.
Therefore, the object of the invention is to provide a device of the aforementioned class that makes it possible to set automatically the printed image to the desired optimal quality.
The invention solves this problem with a device of the aforementioned class by providing at least one camera that scans the printed image on the material web being printed and that feeds images shot in succession to an electronic controller. This controller determines the optimal roller positions from the images that were taken and thus drives automatically the positioning motors.
In this respect it is advantageous to provide a control program that knows the geometric dimensions of the rollers involved in the printing and inking process and that may or may not set tentatively (for example, in the case of long regulating distances or after a roller change) the position of these rollers relative to each other by means of signals to the actuating drives.
However, the inventive process also functions when there is no additional control program.
An advantageous embodiment of the invention provides that the digitized desired contour of the printed image is deposited in the storage unit. This desired contour is then compared (optionally in the controller) with the respective printed image that is shot. Then the controller generates actuating signals for the actuating drives moving the rollers until the comparison yields the best agreement between the printed image that is shot and the stored desired contour.
Another embodiment of the invention does not require a digitized desired contour deposited in a storage. This additional embodiment exploits the fact that the intensity of the reflected light of different segments of the printed image exhibits a characteristic curve as a function of the relative roller position.
Thus, the intensity of the reflected light does not change as long as there is no contact between all of the rollers involved in the printing and inking process. When contact is made, the ink transfer to the material to be printed begins; and the intensity of the reflected light changes quite significantly until the ink transfer reaches an optimal value. As the rollers continue to approach each other, the intensity of the reflected light changes only slightly.
In the area, in which the change in intensity flattens off, an optimum between the ink transfer process and the pressure applied to the rollers to set them in motion in relation to each other is usually reached. If the rollers were to continue to approach each other, the only pressure that would build up would result in the rollers, roller bearings, printing forms, material to be printed, etc. being damaged.
For this reason it is advantageous to section the printed image that is shot into different segments, and to take immediately a picture with a camera that shoots a plurality of image segments.
During the evaluation of the image segments the aforementioned curve of the light intensity is plotted for the individual image segments.
Not until an adequate number of image segments exhibit a certain selected intensity curve is the mutual head-on approach of the rollers terminated. For high requirements posed on the print quality, this requirement will have been met when the change in the intensity of all of the image segments regresses or has already regressed. Thus, it is guaranteed that a good ink transfer to all segments of the printed image takes places.
This embodiment can be improved by forming the difference between the intensity values of the printed material and the intensity values of the non-printed material to be printed. The differentials obtained are called below the contrast values. They can be used in a manner analogous to the intensity values.
The use of at least one color camera is recommended as another advantageous measure, so that light of the selected wavelength ranges can be plotted. This measure is suitable for facilitating the comparison with the stored digitized desired contour of the printed image as well as for improving the curve of the light intensity or the contrast values. Commercial cameras of modern design usually exhibit as the light sensitive elements semiconductor components that are sensitive to light of specific wavelengths, a feature that stems from the photo effect and its application in the semiconductor area. It is advantageous, when a camera is able to assign in this way electrical output values to the color intensity values of several colors (for example, red, yellow, blue). Then these values are made available to a regulating and control unit.
In this way the color intensity curve of different colors or even the entire spectrum of a printed image or even the segments of a printed image can be plotted. Then the measured values are used in the manner described above in order to set the suitable position of the printing rollers. Even for the individual colors a contrast can be formed by the method described above.
Light intensity values can also be transferred into coordinate systems that are appropriate for further evaluation. The same also applies, of course, to the contrast values. These values, derived in the last instance from the intensity values and the color values (wavelength/frequency), also exhibit a characteristic curve as a function of the relative position of the rollers and can be used in the manner described above.
Especially advantageous is the use of the inventive process in flexographic printing, since here the thickness of the blocks must be taken into consideration. In addition, their adhesive strips and the other elements that are involved can exhibit different thickness tolerances so that it can happen that when the parts are set gently into motion so that they just touch, not all of the parts of the blocks produce printed images, thus resulting in only partial images. Therefore, the deviation between the aforementioned geometric desired value and the actual positions of the rollers involved in the printing process is especially large in the case of flexographic printing.
Expediently a digital camera is used as the camera. It delivers digitized images of the printed images that were shot.
In the case of multiple print units each print unit can be set separately.
Furthermore, a separate setting of the actuating drives can be provided in order to make the various rollers parallel, should the pressure differ over the length of a roller on account of its inclined position. In flexographic printing one would provide, for example, for the capability of setting separately the actuating drives of one side of the ink system(s) in order to guarantee, among other things, that the printing and counter-impression cylinder are parallel.
A measurement procedure within the scope of this application is the observation of the course of the intensity and/or contrast values, during which the rollers involved in the printing process are adjusted in relation to each other. If only one camera is used, there is the possibility of adjusting sequentially several inking systems of a machine, that is, of carrying out a measurement process while setting an inking system.
However, it is also possible to carry out only one measurement process at the material to be printed, which has already passed through several inking systems, while adjusting these inking systems of a machine. This procedure results in an additional saving of time. Optionally this procedure is also possible when only one camera is used.
As soon as the setting(s) that yields/yield the best agreement between the printed images that are shot and the desired contour has/have been reached, the values can be deposited in a storage. The same also applies naturally to those set values that are derived from the other setting procedures, according to the invention.
In this way these set values can be found quickly again, for example, after a printing process has been interrupted and after the printing cylinder has been moved away.